Industrial activities generate sediments and sludge that are often disposed of in ponds or impoundments or managed in another fashion. The United States Environmental Protection Agency estimates that, in the 1990s, there were approximately 18,000 industrial surface impoundments that manage sediments in use throughout the United States. These surface impoundments were present at about 7,500 facilities located primarily east of the Mississippi River and in Pacific Coast states.
Surface impoundments are used by many industrial sectors, such as manufacturing, bulk petroleum storage, air and truck transportation, waste management, and national security. The wastewaters and sediments managed in these surface impoundments are primarily from manufacturing and washing processes and certain contaminated stormwaters.
Industrial impoundments vary greatly in size, from less than a quarter of a hectare (⅓ of an acre) to several hundred hectares. The environmental and human health issues plaguing sediment impoundments are well understood. In general, sediment in sediment ponds, such as CCR material, behaves similarly to typical sand, silt and clay sediments, and their geotechnical properties are similar depending on the size and classification. Therefore, the performance of such sediments will be similar and vice versa.
As will be appreciated by those skilled in the art, the properties inherent in CCR residuals such as fly ash, gypsum, calcium sulfite, bottom ash, pyrites and the like are indicative of, and consistent with, the properties of other sediments that may be found in sediment ponds. For example, sediments are removed periodically from the bottom of lakes, rivers harbors and other water bodies as a routine and necessary means to keep our waterway infrastructure operational due to the natural process of sand and silt washing downstream, gradually filling channels and harbors. Once again, these sediments have properties and characteristics that are similar to those described above, and the removal of the dewatering of these sediments is within the scope of the present disclosure.
In one example of the type of sediment contemplated herein, past coal-fired generation activities have resulted in CCR sediments in disposal ponds or impoundments. These CCR ponds require closure to mitigate their impact on the neighboring environment and human or animal health. Closure is also now required by U.S. environmental regulation. However, as is also typical with other sediment ponds, to facilitate closure, the CCR ponds are sometimes dewatered by pre-drainage of the CCR to enhance strength and stability of the material and thereby provide a stable surface on which to operate earthmoving and grading equipment. If pre-drainage (e.g., by pumping wellpoints installed in the CCR to lower the groundwater table) does not sufficiently improve strength and stability of the in-place CCR due to its drainage properties, it becomes necessary to improve CCR strength and stability with admixtures such as quicklime, dry fly ash, or Portland cement; evaporative drying in place, or by dredging or excavating the CCR, dewatering it to consolidate it and improve its strength and handling characteristics, and landfilling it either in the same place or by hauling it a different disposal location.
Sediment ponds are generally known to be unstable when saturated. For example, when saturated, CCR is subject to shear strain, it densifies and expels water, resulting in a near total loss of shear strength. In this state, the material becomes a viscous fluid and may begin to slide or flow. This process may result in overtopping of impoundments and makes excavation and handling difficult to impossible. Reducing the water content in a sediment pond by only a few percentage points has a dramatic effect on its behavior, allowing stable, near vertical cuts suitable for mass excavation. The methods and systems described herein are useful in any sediment pond requiring stabilization by dewatering in order to manage or close the pond.
Dewatering methods include both mechanical dewatering and geotube dewatering. In mechanical dewatering, dredged CCR is pumped to a mechanical dewatering unit (e.g., a centrifuge, a belt press, or a filter press), dewatered, and the filtered CCR (filter cake) is placed in a landfill. Often, the filtered CCR cake requires solidification/stabilization because it cannot support earthwork equipment that is used on the surface of landfills.
Geotube dewatering uses geotubes for dewatering. Geotubes are large filter bags made of geotextile. Dredged CCR is pumped into a geotube and the water is allowed to drain, leaving CCR solids in the geotube. After the geotube is filled with dredged CCR, it is allowed to drain for some time. When the geotube collapses as water is drained, more dredged CCR is pumped into the geotube. After cycles of filling and draining, the geotube is filled with “drained” CCR. The drained CCR may be dewatered further, if desired, by evaporative drying for several weeks. The dewatered CCR may be taken off site for disposal or disposed of in an on-site landfill.
Consolidation refers to a process of subjecting the sediment to a load so that the sediment undergoes volume reduction and strength gain as a result of water being effectively forced out of the loaded sediment volume. Since most sediments do not allow water to flow out easily due to its very low hydraulic conductivity, drainage pathways are provided in the sediment volume to accelerate consolidation.
The most common way of providing drainage pathways is to insert prefabricated drains vertically into sediment in the sediment pond. The prefabricated drains consist of a plastic core wrapped with geotextile filter which, when installed in the sediment, facilitates the flow of water into the drain and to the surface of the ground. Prefabricated drains can consist of flat plastic cores with a geotextile envelope, commonly installed using a hollow rectangular mandrel that is pressed into the ground, or perforated circular plastic pipe/tube surrounded by a geotextile envelope, installed by drilling an open hole with drilling fluid, or jetting or driving an open-ended temporary steel casing/tube or advancing a continuous hollow auger and inserting the perforated plastic pipe or tube and geotextile envelope before the temporary casing/tube or hollow auger is extracted.